Solid-state imaging device, manufacturing method of the same and electronic apparatus

ABSTRACT

Disclosed herein is a solid-state imaging device including: an opto-electrical conversion section provided inside a semiconductor substrate to receive incident light coming from one surface of the semiconductor substrate; a wiring layer provided on the other surface of the semiconductor substrate; and a light absorption layer provided between the other surface of the semiconductor substrate and the wiring layer to absorb transmitted light passing through the opto-electrical conversion section as part of the incident light.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a Continuation application of Ser. No.13/410,911, filed on Mar. 2, 2012, and claims priority based on JapanesePatent Application No. 2011-067076, filed on Mar. 25, 2011, the entirecontents of which are incorporated herein by reference.

BACKGROUND

The present disclosure relates to a solid-state imaging device, itsmanufacturing method and electronic apparatus each making use of thesolid-state imaging device.

An electronic apparatus such as a digital camera employs a solid-stateimaging device. For example, an electronic apparatus employs a CMOS(Complementary Metal Oxide Semiconductor) image sensor serving as thesolid-state imaging device.

The solid-state imaging device includes a plurality of pixels laid outin a pixel area of a semiconductor substrate. Each of the pixels has anopto-electrical conversion section. A typical example of theopto-electrical conversion section is a photodiode in which incidentlight is received by a light receiving surface and subjected to anopto-electrical conversion process in order to convert the light intosignal electric charge.

In a CMOS image sensor, which is a typical solid-state imaging device,each pixel is configured to include a pixel transistor circuit inaddition to an opto-electrical conversion section. The pixel transistorcircuit is configured to read out signal electric charge generated bythe opto-electrical conversion section, and output the signal electriccharge to a signal line as an electrical signal.

In a solid-state imaging device including a plurality of opto-electricalconversion sections laid out on a semiconductor substrate, in general, amulti-layer wiring layer is provided on the front-surface side of thesemiconductor substrate and incident light coming from the front-surfaceside is received by the light receiving surface of the opto-electricalconversion section. A solid-state imaging device having such aconfiguration is referred to as a solid-state imaging device of thefront-surface radiation type. Thus, in the case of the front-surfaceradiation type, the multi-layer wiring layer having a large thicknessexists at a position between a micro-lens and the light receivingsurface. In consequence, wires reduce the aperture ratio. As a result,it may be difficult to improve the sensitivity in some cases.

In order to solve the problem described above, there has been proposed aconfiguration in which incident light comes from the rear-surface sideof the semiconductor substrate is received by the opto-electricalconversion section. The rear-surface side of the semiconductor substrateis a side opposite to the aforementioned front-surface side on which themulti-layer wiring layer is provided. A solid-state imaging devicehaving such a configuration is referred to as a solid-state imagingdevice of the rear-surface radiation type. For more information on therear-surface radiation type, the reader is advised to refer to documentssuch as Japanese Patent Laid-Open Nos. 2010-109295, 2010-186818 and2007-258684.

In the case of the rear-surface radiation type, the thickness of thesemiconductor substrate is reduced to a value of the order of 3 μm. Inconsequence, incident light passing through the semiconductor substratemay be reflected by the wires included in the multi-layer wiring layerand may propagate back to the photodiodes laid out on the semiconductorsubstrate in some cases. Thus, in such cases, a signal generated by aphotodiode includes noises which reduce the quality of the taken image.

SUMMARY

As described above, it is difficult to improve the quality of an imagetaken by making use of the existing imaging device.

It is desirable to provide a solid-state imaging device capable ofimproving the quality of the taken image, a method for manufacturing thesolid-state imaging device and an electronic apparatus employing thesolid-state imaging device.

A solid-state imaging device provided by the present disclosureincludes:

an opto-electrical conversion section provided inside a semiconductorsubstrate to receive incident light coming from one surface of thesemiconductor substrate;

a wiring layer provided on the other surface of the semiconductorsubstrate; and

a light absorption layer provided between the other surface of thesemiconductor substrate and the wiring layer to absorb transmitted lightpassing through the opto-electrical conversion section as part of theincident light.

An electronic apparatus provided by the present disclosure employs thesolid-state imaging device described above.

A method for manufacturing a solid-state imaging device provided by thepresent disclosure includes:

providing an opto-electrical conversion section inside a semiconductorsubstrate to serve as a section for receiving incident light coming fromone surface of the semiconductor substrate;

providing a light absorption layer on the other surface of thesemiconductor substrate to serve as a layer for absorbing transmittedlight passing through the opto-electrical conversion section as part ofthe incident light; and

providing a wiring layer so as to cover the other surface pertaining tothe semiconductor substrate to serve as a surface on which the lightabsorption layer has been provided.

In accordance with the present disclosure, the light absorption layerprovided between the other surface of the semiconductor substrate andthe wiring layer absorbs transmitted light passing through theopto-electrical conversion section as part of the incident light.

In accordance with the present disclosure, it is possible to provide asolid-state imaging device capable of improving the quality of the takenimage, a method for manufacturing the solid-state imaging device and anelectronic apparatus employing the solid-state imaging device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the configuration of a camera according to afirst embodiment;

FIG. 2 is a diagram showing the entire configuration of a solid-stateimaging device according to the first embodiment;

FIG. 3 is a diagram showing principal elements of the solid-stateimaging device according to the first embodiment;

FIG. 4 is a diagram showing principal elements of the solid-stateimaging device according to the first embodiment;

FIG. 5 is a diagram showing principal elements of the solid-stateimaging device according to the first embodiment;

FIG. 6 is a diagram showing principal elements of the solid-stateimaging device according to the first embodiment;

FIG. 7 is a diagram showing a color filter employed in the firstembodiment;

FIGS. 8A to 8C are timing charts of control signals supplied to a pixeltransistor circuit of a pixel in an operation to take an image in thefirst embodiment;

FIGS. 9A to 9D are a plurality of diagrams showing a method formanufacturing the solid-state imaging device according to the firstembodiment;

FIG. 10 is a diagram showing principal elements of a solid-state imagingdevice according to a modified version of the first embodiment;

FIG. 11 is a diagram showing principal elements of a solid-state imagingdevice according to a second embodiment; and

FIG. 12 is a diagram showing principal elements of the solid-stateimaging device according to the second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present disclosure are explained byreferring to accompanying diagrams as follows.

It is to be noted that the description of the embodiments is dividedinto topics arranged in the following order.

1: First Embodiment (The light absorption layer is the same layer asgates)2: Second Embodiment (The light absorption layer is a layer differentfrom that of gates)

3: Others 1: First Embodiment (A) Apparatus Configuration (A-1) CameraPrincipal-Element Configuration

FIG. 1 is a diagram showing the configuration of a camera 40 accordingto a first embodiment.

As shown in FIG. 1, the camera 40 has a solid-state imaging device 1, anoptical system 42, a control section 43 and a signal processing section44. Each of them is explained sequentially one after another as follows.

Incident light H coming from an imaging object through the opticalsystem 42 is received by the imaging surface PS of the solid-stateimaging device 1 and the solid-state imaging device 1 carries out anopto-electrical conversion process on the light H in order to generatesignal electric charge. Then, the solid-state imaging device 1 reads outthe signal electric charge obtained as a result of the opto-electricalconversion process and outputs the signal electric charge as anelectrical signal to the signal processing section 44. The solid-stateimaging device 1 carries out these operations in accordance with acontrol signal received from the control section 43.

The optical system 2 includes optical members such as an image creationlens and a stop. The optical system 2 converges the incident light H onthe imaging surface PS of the solid-state imaging device 1.

The control section 43 outputs a variety of control signals to thesolid-state imaging device 1 and the signal processing section 44 inorder to control and drive the solid-state imaging device 1 and thesignal processing section 44.

The signal processing section 44 processes the electrical signalreceived from the solid-state imaging device 1 as raw data in order togenerate a digital image of the imaging object.

(A-2) Configuration of Principal Elements of the Solid-State ImagingDevice

The entire configuration of the solid-state imaging device 1 isexplained as follows.

FIG. 2 is a diagram showing the entire configuration of the solid-stateimaging device 1 according to the first embodiment.

As shown in FIG. 2, the solid-state imaging device 1 according to thisembodiment includes a semiconductor substrate 101. The semiconductorsubstrate 101 is typically a single-crystal silicon substrate having asmall film thickness. The surface of the semiconductor substrate 101includes a pixel area PA and a surrounding area SA.

As shown in FIG. 2, the pixel area PA has a rectangular shape andincludes a plurality of pixels P laid out in the horizontal direction xand the vertical direction y. That is to say, the pixels P are laid outto form a matrix.

Each of the pixels P in the pixel area PA is configured to receiveincident light and convert the light into signal electric charge. Then,a pixel transistor circuit not shown in the figure reads out the signalelectric charge and outputs the signal electric charge to the signalprocessing section 44 as an electrical signal. A detailed configurationof the pixel P will be described later.

As shown in FIG. 2, the surrounding area SA is placed at locationssurrounding the pixel area PA. In the surrounding area SA, peripheralcircuits are provided.

To put it concretely, as shown in FIG. 2, the peripheral circuitsinclude a vertical driving circuit 13, a column circuit 14, a horizontaldriving circuit 15, an external-destination output circuit 17, a TG(timing generator) 18 and a shutter driving circuit 19.

As shown in FIG. 2, in the surrounding area SA, the vertical drivingcircuit 13 is provided on a side of the pixel area PA. The verticaldriving circuit 13 is configured to select and drive the pixels P in thepixel area PA in row units.

As shown in FIG. 2, in the surrounding area SA, the column circuit 14 isprovided beneath the pixel area PA. The column circuit 14 is configuredto carry out signal processing on electrical signals received from thepixels P in column units. The column circuit 14 includes a CDS(Correlated Double Sampling) circuit not shown in the figure and carriesout the signal processing also in order to remove fixed-pattern noises.

As shown in FIG. 2, the horizontal driving circuit 15 is electricallyconnected to the column circuit 14. The horizontal driving circuit 15typically includes a shift register and sequentially provides theexternal-destination output circuit 17 with signals each held in thecolumn circuit 14 for one of the columns of the matrix of the pixels P.

As shown in FIG. 2, the external-destination output circuit 17 is alsoelectrically connected to the column circuit 14. Theexternal-destination output circuit 17 carries out signal processing onelectrical signals received from the column circuit 14 and outputsdigital signals obtained as a result of the signal processing to thesignal processing section 44 external to the solid-state imaging device1. The external-destination output circuit 17 includes an AGC (AutomaticGain Control) circuit 17 a and an ADC (Analog-to-Digital Conversion)circuit 17 b. In the external-destination output circuit 17, the AGCcircuit 17 a applies a gain to an analog electrical signal and, then,the ADC circuit 17 b converts the analog electrical signal into thedigital signal, outputting the digital signal to the signal processingsection 44 external to the solid-state imaging device 1.

As shown in FIG. 2, the time generator 18 is electrically connected tothe vertical driving circuit 13, the column circuit 14, the horizontaldriving circuit 15, the external-destination output circuit 17 and theshutter driving circuit 19. The time generator 18 generates a variety oftiming signals, supplying the timing signals to the vertical drivingcircuit 13, the column circuit 14, the horizontal driving circuit 15,the external-destination output circuit 17 and the shutter drivingcircuit 19 in order to drive and control each circuit.

The shutter driving circuit 19 is configured to select the pixels P inrow units and adjust the exposure time for the selected pixels P.

(A-3) Detailed Configuration of the Solid-State Imaging Device

A detailed configuration of the solid-state imaging device according tothis embodiment is explained as follows.

FIGS. 3 to 6 are each a diagram showing principal elements employed inthe solid-state imaging device 1 according to the first embodiment.

To be more specific, FIG. 3 is a diagram showing a cross section of thesolid-state imaging device 1 including a semiconductor substrate 101.FIGS. 4 and 5 are each a diagram showing the surface of thesemiconductor substrate 101 shown in FIG. 3, that is, showing a lowersurface shown in FIG. 3. FIG. 6 is a diagram showing the circuitconfiguration of the pixel P.

To put it in detail, FIG. 3 is a diagram showing a model representing across section of a portion X1-X2 shown in FIGS. 4 and 5. FIG. 4 is adiagram showing the lower surface shown in FIG. 3 as the front surfaceof the semiconductor substrate 101 except a support substrate SS, awiring layer 111, a light-reflection preventing film 301 and a lightabsorption layer 401. On the other hand, FIG. 5 is a diagram showing thelower surface shown in FIG. 3 as the front surface of the semiconductorsubstrate 101 with the light absorption layer 401 provided thereon asshown by a hatched portion. It is to be noted that, for the sake ofdrawing convenience, in the figures, the shape (including the width) ofeach portion may be properly changed from diagram to diagram.

As shown in FIG. 3, the solid-state imaging device 1 includes aphotodiode 21 provided inside the semiconductor substrate 101. Thesemiconductor substrate 101 is a substrate made from a single-crystalsilicon having a small film thickness.

As shown in FIG. 3, on an upper surface shown in FIG. 3 as the rearsurface of the semiconductor substrate 101, there are provided a lightshielding layer 122, a color filter CF and a microlens ML.

On a lower surface shown in FIG. 3 as the front surface of thesemiconductor substrate 101, on the other hand, a transfer transistor 22is provided. As shown in FIGS. 4 and 5, in addition to the transfertransistor 22, there are also provided an amplification transistor 23, aselect transistor 24 and a reset transistor 25. The transfer transistor22, the amplification transistor 23, the select transistor 24 and thereset transistor 25 compose a pixel transistor circuit Tr. However, theamplification transistor 23, the select transistor 24 and the resettransistor 25 are not shown in FIG. 3.

In addition, as shown in FIG. 3, members including the light absorptionlayer 401 are also provided on a lower surface shown in FIG. 3 as thefront surface of the semiconductor substrate 101. Furthermore, on alower surface shown in FIG. 3 as the front surface of the semiconductorsubstrate 101, the wiring layer 111 is also provided. Moreover, on aspecific side of the wiring layer 111, the support substrate SS isprovided. The specific side of the wiring layer 111 is a side oppositeto the other side close to the semiconductor substrate 101.

That is to say, the solid-state imaging device 1 according to theembodiment is configured to work as a CMOS image sensor of therear-surface radiation type. In the case of the rear-surface radiationtype, incident light H coming from the rear surface which is an uppersurface shown in FIG. 3 is received by the photodiode 21 and convertedthereby into signal electric charge in an operation to take a colorimage.

Details of every section employed in the solid-state imaging device 1are explained as follows.

(a) Photodiode 21

In the solid-state imaging device 1, a plurality of photodiodes 21 arelaid out in the pixel area PA so that each of the photodiodes 21 isassociated with one of a plurality of the pixels P shown in FIG. 2. Thatis to say, the photodiodes 21 are laid out on the imaging plane which isan xy plane in the horizontal direction x and the vertical direction yperpendicular to the horizontal direction x.

As described above, the photodiode 21 is configured to receive incidentlight H, carry out an opto-electrical conversion process to convert theincident light H into signal electric charge and then store the electriccharge.

In this case, as shown in FIG. 3, the photodiode 21 receives theincident light H coming from a side close to an upper surface shown inFIG. 3 as the rear surface of the semiconductor substrate 101. As shownin FIG. 3, the color filter CF and the microlens ML are provided abovethe photodiode 21. Thus, the incident light H propagates to thephotodiode 21 by sequentially passing through the microlens ML and thecolor filter CF, being received by a light receiving surface JS of thephotodiode 21. Then, the photodiode 21 carries out an opto-electricalconversion process on the incident light H in order to convert theincident light H into signal electric charge as described above.

As shown in FIG. 3, the photodiode 21 is provided inside thesemiconductor substrate 101.

Typically, an n-type semiconductor area 101 n is created as anelectric-charge accumulation area used for accumulating electronsserving as electric charge. In the photodiode 21, the n-typesemiconductor area 101 n is sandwiched by a p-type semiconductor area101 pa provided on a side close to an upper surface shown in FIG. 3 asthe rear surface of the n-type semiconductor area 101 n and a p-typesemiconductor area 101 pc provided on a side close to a lower surfaceshown in FIG. 3 as the front surface of the n-type semiconductor area101 n. That is to say, the photodiode 21 has a HAD (Hole AccumulationDiode) structure in which the p-type semiconductor areas 101 pa and 101pc are created in order to prevent dark currents from being generated inboundary faces on the upper-surface and lower-surface sides of then-type semiconductor area 101 n. The signal electric charge generated bythe photodiode 21 drifts inside the n-type semiconductor area 101 n andis accumulated in the vicinity of the p-type semiconductor area 101 pcprovided on a side close to a lower surface shown in FIG. 3 as the frontsurface of the n-type semiconductor area 101 n.

As shown in FIGS. 3 and 4, a pixel separation section PB is providedbetween pixels P. The photodiode 21 is provided in an area laid outbetween the pixel separation sections PB as an area inside the pixel P.To put it concretely, as shown in FIG. 3, on the pixel separationsection PB, a p-type semiconductor area 101 pb is provided, beingextended through the inside of the semiconductor substrate 101 from anupper surface shown in FIG. 3 as the rear surface of the semiconductorsubstrate 101 to a lower surface shown in FIG. 3 as the front surface ofthe semiconductor substrate 101. In this way, pixels P are electricallyseparated from each other. As shown in FIG. 4, the pixel separationsections PB are provided to form a grid existing between pixels P. Thephotodiode 21 is provided in an area laid out between the pixelseparation sections PB forming the grid as an area inside the pixel P.

As shown in FIG. 6, the anode of the photodiode 21 is connected to theground. Electrons are accumulated in the photodiode 21 as signalelectric charge which is read out by the pixel transistor circuit Tr.The pixel transistor circuit Tr outputs the signal electric charge to avertical signal line 27 as an electrical signal.

(b) Light Shielding Layer 122

In the solid-state imaging device 1, as shown in FIG. 3, the lightshielding layer 122 is provided on a side close to an upper surfaceshown in FIG. 3 as the rear surface of the semiconductor substrate 101.

In this configuration, as shown in FIG. 3, an insulation layer 121 isprovided so as to cover an upper surface shown in FIG. 3 as the rearsurface of the semiconductor substrate 101 whereas the light shieldinglayer 122 is provided on the upper surface of the insulation layer 121.On an upper surface shown in FIG. 3 as the rear surface of thesemiconductor substrate 101, the light shielding layer 122 has portionscorresponding to the pixel separation sections PB. However, the lightshielding layer 122 does not have portions corresponding to the pixelsP. That is to say, on the light receiving surface JS of the photodiode21, an aperture exists.

It is to be noted that, though not shown in FIG. 4, the planar shape ofthe light shielding layer 122 is the shape of a grid similar to thatformed by the pixel separation sections PB.

In this embodiment, the insulation layer 121 shown in FIG. 3 is madefrom typically SiO₂ (a silicon oxide). On the other hand, the lightshielding layer 122 is made from typically a metallic material such as W(tungsten). The light shielding layer 122 can also be made properly fromAl (aluminum) in place of W (tungsten).

Then, on the upper surface of the insulation layer 121, a flatteningfilm 123 is provided so as to cover the light shielding layer 122. Theflattening film 123 is made from an optically transparent material.

(c) Color Filter CF

In the solid-state imaging device 1, as shown in FIG. 3, the colorfilter CF is provided on the upper surface of the flattening film 123 ona side close to an upper surface shown in FIG. 3 as the rear surface ofthe semiconductor substrate 101.

FIG. 7 is a diagram showing a color filter CF employed in the firstembodiment. To be more specific,

FIG. 7 shows a top view of the color filter CF.

As shown in FIG. 7, the color filter CF includes typically a red-colorfilter layer CFR, a green-color filter layer CFG and a blue-color filterlayer CFB. The red-color filter layer CFR, the green-color filter layerCFG and the blue-color filter layer CFB are placed at positions adjacentto each other. Each of the red-color filter layer CFR, the green-colorfilter layer CFG and the blue-color filter layer CFB is associated withone of the pixels P.

In this case, as shown in FIG. 7, the red-color filter layer CFR, thegreen-color filter layer CFG and the blue-color filter layer CFB arelaid out to form typically a Bayer array. That is to say, a plurality ofaforementioned green-color filter layers CFG are laid out in a diagonaldirection to form a checker board design. Then, a red-color filter layerCFR and a blue-color filter layer CFB are separated away from each otherin another diagonal direction to form a square block in conjunction withtwo green-color filter layers CFG adjacent to each other.

The red-color filter layer CFR of the color filter CF is created to havea high optical transmittance for light having a wavelength in awavelength band of typically 625 to 740 nm for the red color. That is tosay, the red-color filter layer CFR is created so as to allow theincident light H to pass through the red-color filter layer CFR andpropagate to the light receiving surface JS as red-color light.

By the same token, the green-color filter layer CFG of the color filterCF is created to have a high optical transmittance for light having awavelength in a wavelength band of typically 500 to 565 nm for the greencolor. Thus, the green-color filter layer CFG has a high opticaltransmittance for light having a wavelength shorter than the wavelengthband of light passing through the red-color filter layer CFR. That is tosay, the green-color filter layer CFG is created so as to allow theincident light H to pass through the green-color filter layer CFG andpropagate to the light receiving surface JS as green-color light.

In the same way, the blue-color filter layer CFB of the color filter CFis created to have a high optical transmittance for light having awavelength in a wavelength band of typically 450 to 485 nm for the bluecolor. Thus, the blue-color filter layer CFB has a high opticaltransmittance for light having a wavelength shorter than the wavelengthband of light passing through the green-color filter layer CFG. That isto say, the blue-color filter layer CFB is created so as to allow theincident light H to pass through the blue-color filter layer CFB andpropagate to the light receiving surface JS as blue-color light.

As described above, the red-color filter layer CFR, the green-colorfilter layer CFG and the blue-color filter layer CFB which compose thecolor filter CF have high optical transmittances for light rays ofhaving wavelengths in wavelength bands different from each other. Thered-color filter layer CFR, the green-color filter layer CFG and theblue-color filter layer CFB are each associated with one of the pixels Pand laid out at positions adjacent to each other.

(d) Microlens ML

As shown in FIG. 3, in the solid-state imaging device 1, the microlensML is provided on the upper surface of the color filter CF provided on aside close to an upper surface shown in FIG. 3 as the rear surface ofthe semiconductor substrate 101.

The solid-state imaging device 1 has a plurality of such microlenses MLlaid out at positions each corresponding to one of the pixels P.Provided on a side close to an upper surface shown in FIG. 3 as the rearsurface of the semiconductor substrate 101, the microlens ML is a convexlens having a shape protruding in the upward direction. The microlens MLis configured to converge the incident light H on the photodiode 21 ofevery pixel P. Typically, the microlens ML is made from transparentresin such as the styrene resin, the acryl resin or the novolac resin.

(e) Pixel Transistor Circuit Tr

In the solid-state imaging device 1, a plurality of pixel transistorcircuits Tr are provided so that each of the pixel transistor circuitsTr is associated with one of the pixels P shown in FIG. 2.

As shown in FIGS. 4 to 6, the pixel transistor circuit Tr includes thetransfer transistor 22, the amplification transistor 23, the selecttransistor 24 and the reset transistor 25. The pixel transistor circuitTr is configured to read out signal electric charge from the photodiode21 and output the electric charge as an electrical signal.

FIG. 3 shows the transfer transistor 22 of the pixel transistor circuitTr. In the same way as the transfer transistor 22, however, theamplification transistor 23, the select transistor 24 and the resettransistor 25 are provided on a lower surface shown in the figure as thefront surface of the semiconductor substrate 101. A gate electrode 22Gof the transfer transistor 22 employed in the pixel transistor circuitTr is created from typically silicon through typically a gate insulationfilm 110 which is a silicon-oxide film. By the same token, though notshown in FIG. 3, the gate electrodes of the amplification transistor 23,the select transistor 24 and the reset transistor 25 are each createdfrom typically silicon through typically the gate insulation film 110which is a silicon-oxide film. The transistors 22 to 25 are each anN-channel MOS transistor. In addition, on a lower surface shown in thefigure as the front surface of the semiconductor substrate 101, thetransistors 22 to 25 are covered by the wiring layer 111.

As shown in FIGS. 4 and 5, in the pixel transistor circuit Tr, thetransfer transistor 22 is provided at a location close to the photodiode21. In this configuration, as shown in FIG. 6, the transfer transistor22 is provided at a location between the photodiode 21 and a floatingdiffusion FD so that the transfer transistor 22 is capable oftransferring signal electric charge from the photodiode 21 to thefloating diffusion FD. To put it concretely, the gate electrode of thetransfer transistor 22 is electrically connected to a transfer line 26.On the basis of a transfer signal TG supplied from the transfer line 26to the gate electrode of the transfer transistor 22, the transfertransistor 22 transfers signal electric charge accumulated in thephotodiode 21 to the floating diffusion FD.

As shown in FIGS. 4 and 5, the amplification transistor 23 employed inthe pixel transistor circuit Tr is provided at a location sandwichedbetween the select transistor 24 and the reset transistor 25 in thepixel separation section PB. As shown in FIG. 6, the amplificationtransistor 23 is configured to amplify an input electrical signalappearing at the floating diffusion FD and output the amplified signal.The input electrical signal is a voltage obtained as a result of aconversion process carried out on the signal electric charge in order toconvert the electrical charge into the voltage. To put it concretely,the gate electrode of the amplification transistor 23 is electricallyconnected to the floating diffusion FD. In addition, the drain electrodeof the amplification transistor 23 is electrically connected to apower-supply supply line Vdd whereas the source electrode of theamplification transistor 23 is electrically connected to the selecttransistor 24. When the select transistor 24 is selected and put in aturned-on state by a select signal SEL supplied to the gate electrode ofthe select transistor 24, a constant-current source I supplies aconstant current to the amplification transistor 23 through the selecttransistor 24 so that the amplification transistor 23 operates as asource follower. Thus, by supplying the select signal SEL to the gateelectrode of the select transistor 24, the amplification transistor 23amplifies the electrical signal which is a voltage obtained as a resultof a conversion process carried out in the floating diffusion FD on thesignal electric charge in order to convert the electrical charge intothe voltage.

As shown in FIGS. 4 and 5, the select transistor 24 employed in thepixel transistor circuit Tr is provided to line up with theamplification transistor 23 and the reset transistor 25 in a pixelseparation section PB. In this configuration, with the select transistor24 selected by the select signal SEL, as shown in FIG. 6, the selecttransistor 24 passes on the electrical signal, which is output by theamplification transistor 23, to the vertical signal line 27. To put itconcretely, the gate electrode of the select transistor 24 iselectrically connected to an address line 28 conveying the select signalSEL. When the select signal SEL is supplied to the gate electrode of theselect transistor 24, the select transistor 24 is put in a turned-onstate, passing on the electrical signal, which is output by theamplification transistor 23 as a result of amplification, to thevertical signal line 27.

As shown in FIGS. 4 and 5, the reset transistor 25 employed in the pixeltransistor circuit Tr is provided to line up with the amplificationtransistor 23 and the select transistor 24 in a pixel separation sectionPB. As shown in FIG. 6, the reset transistor 25 is configured to resetan electric potential appearing on the gate electrode of theamplification transistor 23. To put it concretely, the gate electrode ofthe reset transistor 25 is electrically connected to a reset line 29conveying a reset signal RST. In addition, the drain electrode of thereset transistor 25 is electrically connected to the power-supply supplyline Vdd whereas the source electrode of the reset transistor 25 iselectrically connected to the floating diffusion FD. On the basis of thereset signal RST supplied from the reset line 29 to the reset transistor25, the reset transistor 25 resets the electric potential appearing onthe floating diffusion FD electrically connected to the gate electrodeof the amplification transistor 23 to the voltage of a power supply.

FIGS. 8A to 8C are timing charts of control signals supplied to thepixel transistor circuit Tr of a pixel P in an operation to take animage in the first embodiment.

To be more specific, FIG. 8A shows the timing chart of the select signalSEL supplied to the gate electrode of the select transistor 24 whereasFIG. 8B shows the timing chart of the reset signal RST supplied to thegate electrode of the reset transistor 25. On the other hand, FIG. 8Cshows the timing chart of the transfer signal TG supplied to the gateelectrode of the transfer transistor 22 (refer to FIG. 6).

As shown in FIGS. 8A to 8C, when an operation to take an image iscarried out, at a first time point t1, the select signal SEL is raisedto a high level in order to put the select transistor 24 in a turned-onstate. Then, at a second time point t2, the reset signal RST is raisedto a high level in order to put the reset transistor 25 in a turned-onstate. Thus, the electric potential appearing on the gate electrode ofthe amplification transistor 23 is reset (refer to FIG. 6).

Subsequently, as shown in FIGS. 8A to 8C, at a third time point t3, thereset signal RST is pulled down to a low level in order to put the resettransistor 25 in a turned-off state. Then, later on, a voltagerepresenting a reset level is read out and supplied to the columncircuit 14 as an output signal (refer to FIGS. 2 and 6).

Subsequently, as shown in FIGS. 8A to 8C, at a fourth time point t4, thetransfer signal TG is raised to a high level in order to put thetransfer transistor 22 in a turned-on state. For example, a negativevoltage has been applied to the gate electrode of the transfertransistor 22 in order to put the transfer transistor 22 in a turned-offstate during an electric-charge accumulation period. With the transfertransistor 22 put in a turned-off state, the transfer signal TG raisedto a high level to become a positive voltage is applied to the gateelectrode of the transfer transistor 22 in order to put the transfertransistor 22 in a turned-on state. Thus, signal electric chargeaccumulated in the photodiode 21 during the electric-charge accumulationperiod is transferred to the floating diffusion FD (refer to FIG. 6).

Then, as shown in FIGS. 8A to 8C, at a fifth time point t5, the transfersignal TG is pulled down to a low level in order to put the transfertransistor 22 in a turned-off state. Thereafter, the voltagerepresenting a signal level according to the signal electric chargeaccumulated in the photodiode 21 is supplied to the column circuit 14 asan output signal (refer to FIGS. 2 and 6). Subsequently, the selectsignal SEL is pulled down to a low level in order to put the selecttransistor 24 in a turned-off state.

The column circuit 14 carries out differential processing on the signalsupplied thereto earlier to represent the reset level and the signalsupplied thereto later to represent the signal level according to thesignal electric charge accumulated in the photodiode 21. Then, thecolumn circuit 14 stores a signal obtained as a result of thedifferential processing (refer to FIG. 2). It is thus possible to cancelfixed-pattern noises generated by, among others, variations of thethreshold voltages Vth of the transistors provided in every pixel P frompixel P to pixel P.

As described above, operations to drive the pixels P are carried out inrow units each including a plurality of pixels P laid out in thehorizontal direction x. The gate electrodes of the transfer transistors22 each employed in one of the pixels P provided on a row are connectedto each other by a transfer line 26 provided for the row. By the sametokens, the gate electrodes of the select transistors 24 each employedin one of the pixels P provided on a row are connected to each other byan address line 28 provided for the row. In the same way, the gateelectrodes of the reset transistors 25 each employed in one of thepixels P provided on a row are connected to each other by a reset line29 provided for the row. Thus, an operation to drive the pixels Pprovided on a row can be carried out at the same time for the pixels Pon the row.

To put it concretely, the select signal SEL supplied by the verticaldriving circuit 13 described earlier selects a horizontal line referredto as a pixel row from pixel rows in the matrix sequentially one rowafter another in the vertical direction y. Then, a variety of timingsignals generated by the time generator 18 control the transistorsemployed in each of the pixels P. Thus, for every column of the pixelsP, an electrical signal generated in each of the pixels P is supplied tothe column circuit 14 to be stored therein through a vertical signalline 27 provided for the column. Subsequently, the horizontal drivingcircuit 15 selects a signal stored in the column circuit 14 and outputsthe selected signals sequentially to the external-destination outputcircuit 17 (refer to FIGS. 2 and 6).

Then, the signal processing section 44 carries out signal processing onsignals obtained from the operation to take an image by handling thesignals as raw data in order to generate a digital image (refer to FIG.1).

(f) Light-Reflection Preventing Film 301 and Light Absorption Layer 401

As shown in FIG. 3, in the solid-state imaging device 1, each of thelight-reflection preventing film 301 and the light absorption layer 401is provided on a lower surface shown in the figure as the front surfaceof the semiconductor substrate 101.

In this configuration, as shown in FIG. 3, the gate insulation film 110is provided so as to cover the entire lower surface shown in the figureas the front surface of the semiconductor substrate 101. Then, thelight-reflection preventing film 301 is provided on the gate insulationfilm 110 to form a stack in conjunction with the gate insulation film110. The light-reflection preventing film 301 is provided on the samelayer as the gate electrode 22G without covering the gate electrode 22G.

The incident light H coming from an imaging object includes lightcapable of passing through the photodiode 21. The light-reflectionpreventing film 301 is configured to prevent the light passing throughthe photodiode 21 from being reflected on the boundary face between thegate insulation film 110 and the light absorption layer 401.

To put it concretely, the light-reflection preventing film 301 iscreated by setting the film thickness thereof at a proper value from aproperly selected material so as to allow a light-reflection preventingfunction to be implemented on the basis of an optical interferenceeffect. The properly selected material used for making thelight-reflection preventing film 301 is an insulation material having arefraction index between the refraction index of a material used formaking the gate insulation film 110 and the refraction index of amaterial used for making the light absorption layer 401.

As shown in FIG. 3, the light absorption layer 401 is provided on thelight-reflection preventing film 301 created on the gate insulation film110 provided on a lower surface shown in the figure as the front surfaceof the semiconductor substrate 101. The light absorption layer 401 isprovided on the same layer as the gate electrode 22G of the transfertransistor 22 without covering the gate electrode 22G.

As shown in FIG. 5, the light absorption layer 401 is created below alower surface shown in the FIG. 3 as the front surface of thesemiconductor substrate 101 so as to cover a portion at which thephotodiode 21 is provided. However, the light absorption layer 401provided below a lower surface shown in the FIG. 3 as the front surfaceof the semiconductor substrate 101 does not cover a portion at which thepixel transistor circuit Tr including the transfer transistor 22 isprovided. The light absorption layer 401 is provided as a singlejunction between pixels P lined up in the vertical direction y.

In addition, as shown in FIG. 5, the light absorption layer 401 iscreated so that the width D of a gap between the light absorption layer401 and the gate electrodes of the transistors 22 to 25 which areprovided on the same layer is equal to or greater than 0.1 μm. It isthus possible to prevent a bad electrical effect from being generated inthe gap between the light absorption layer 401 and the gate electrodesof the transistors 22 to 25. These gate electrodes include the gateelectrode 22G shown in FIG. 3 as the gate electrode of the transfertransistor 22.

As described above, the incident light H coming from an imaging objectincludes light capable of passing through the photodiode 21. The lightabsorption layer 401 is configured to absorb the light capable ofpassing through the photodiode 21. To put it in detail, the incidentlight H coming from a side close to a upper surface shown in the FIG. 3as the rear surface of the semiconductor substrate 101 includes lightcapable of passing through the photodiode 21 as light other than lightabsorbed by the photodiode 21. The light absorption layer 401 isconfigured to absorb the light capable of passing through the photodiode21 in order to block this light before this light attains the wiringlayer 111.

To put it concretely, the light absorption layer 401 is created bymaking use of a material having a light absorption coefficient greaterthan the single-crystal silicon serving as a material used for makingthe semiconductor substrate 101. For example, the light absorption layer401 is created by making use of the amorphous silicon. Even though FIG.5 does not show the light-reflection preventing film 301, thelight-reflection preventing film 301 has the same planar shape as thelight absorption layer 401.

(g) Wiring Layer 111

In the solid-state imaging device 1, as shown in FIG. 3, the wiringlayer 111 is provided on a lower surface shown in the figure as thefront surface of the semiconductor substrate 101.

As shown in FIG. 3, the wiring layer 111 includes wires 111 h and aninsulation layer 111 z which is an inter-layer insulation film. Thewiring layer 111 is the so-called multi-layer wiring layer which is astack constructed by alternately creating the inter-layer insulationfilm serving as the insulation layer 111 z and the wires 111 h aplurality of times. Typically, the wires 111 h are created from ametallic material such as the aluminum. On the other hand, theinsulation layer 111 z is typically created from an insulation materialsuch as a silicon oxide.

In the wiring layer 111, the wires 111 h are created betweenaforementioned insulation layers 111 z to electrically connect elementsto each other. In the wiring layer 111, each wiring is electricallyconnected the pixel transistor circuit Tr, for example. The wires 111 helectrically connect the transfer transistor 22, the amplificationtransistor 23, the select transistor 24 and the reset transistor 25 toeach other through contacts CON. That is to say, the wiring layer 111 isconstructed as a stack by alternately creating the insulation layer 111z and the wires 111 h a plurality of times so that the wires 111 hfunction as a variety of wires such as the transfer line 26, thevertical signal line 27, the address line 28 and the reset line 29 whichare shown in FIG. 6.

In addition, the wires 111 h are created so that the width of the gapbetween the wires 111 h and the light absorption layer 401 is equal toor greater than 0.1 μm. It is thus possible to prevent a bad electricaleffect from being generated in the gap between the light absorptionlayer 401 and the wires 111 h.

Then, on a surface on a specific side of the wiring layer 111, a supportsubstrate SS is provided. This specific side is opposite to a side onwhich the semiconductor substrate 101 is provided. The support substrateSS is made from a silicon semiconductor and has a typical thickness ofseveral hundreds of μm.

(B) Manufacturing Method

Next, principal elements of a method for manufacturing the solid-stateimaging device 1 are explained as follows.

FIGS. 9A to 9D are a plurality of diagrams showing the method formanufacturing the solid-state imaging device 1 according to the firstembodiment.

Each of FIGS. 9A to 9D is a cross-sectional diagram like FIG. 3.

FIGS. 9A to 9D are referred to in description of processes (a) to (d)carried out to manufacture the solid-state imaging device 1 shown indiagrams such as FIG. 3 as a device according to the embodiment.

Processes shown in FIGS. 9A to 9D are explained sequentially in detailas follows.

(a) Creation of Elements Including the Photodiode 21 (FIG. 9A)

First of all, members including the photodiode 21 are created as shownin FIG. 9A.

In this process, from an upper surface shown in FIG. 9A as the frontsurface of the semiconductor substrate 101 made from a single-crystalsilicon semiconductor, ions each serving as an impurity are injected inorder to create members including the photodiode 21. To put itconcretely, the p-type semiconductor areas 101 pa, 101 pb and 101 pc aswell as the n-type semiconductor area 101 n are created in thesemiconductor substrate 101.

(b) Creation of Elements Including the Transfer Transistor 22 (FIG. 9B)

Then, members including the transfer transistor 22 are created as shownin FIG. 9B.

In this process, typically, thermal oxidation method is adopted in orderto create a silicon-oxide film on an entire upper surface shown in FIG.9B as the front surface of the semiconductor substrate 101 so as toprovide the gate insulation film 110.

Then, the gate electrode 22G is created on an upper surface shown inFIG. 9B as the front surface of the gate insulation film 110. In theprocess of creating the gate electrode 22G, a film is formed from aconductive material typically under conditions described below in orderto provide a conductive layer not shown in the figure. Subsequently, apattern creation work is carried out on the conductive layer not shownin the figure in order to form the gate electrode 22G.

Conditions

Material: Polysilicon

Thickness: 100 to 300 nm

Film creation method: CVD method

The gate electrodes of the transistors 23 to 25 which are other than thetransfer transistor 22 are also created in the same way as the gateelectrode 22G of the transfer transistor 22. Then, the source and drainelectrodes of the transistors 22 to 25 are created. In this way, thepixel transistor circuit Tr is created on an upper surface shown inFIGS. 9A to 9D as the front surface of the semiconductor substrate 101.

(c) Creation of the Light-Reflection Preventing Film 301 and the LightAbsorption Film 401 (FIG. 9C)

Next, the light-reflection preventing film 301 and the light absorptionlayer 401 are created as shown in FIG. 9C.

In this process, the light-reflection preventing film 301 is created onan upper surface shown in FIG. 9C as the front surface of the gateinsulation film 110. Then, the light absorption layer 401 is created onan upper surface shown in FIG. 9C as the front surface of thelight-reflection preventing film 301.

The light-reflection preventing film 301 is created by setting the filmthickness thereof at a proper value from a properly selected material soas to allow a light-reflection preventing function to be implemented onthe basis of an optical interference effect. The properly selectedmaterial used for making the light-reflection preventing film 301 is aninsulation material having a refraction index between the refractionindex of a material used for making the gate insulation film 110 and therefraction index of a material used for making the light absorptionlayer 401.

For example, it is preferable to create the light-reflection preventingfilm 301 from a material with its refraction index n0 satisfying anequation described below. Thus, since incident light having a wavelengthλ can be cancelled by interferences, the light-reflection preventingfunction can be well implemented. In the following equation, notation n1denotes the refraction index of the light absorption layer 401positioned above the light-reflection preventing film 301 whereasnotation n2 denotes the refraction index of the silicon semiconductorsubstrate 101 (Si) positioned below the light-reflection preventing film301. The refraction index n2 of the silicon semiconductor substrate 101is 4.2 (that is, n2=4.2).

(n2−n0)²/(n2+n0)²=(n0−n1)²/(n0+n1)²

That is to say, let the light absorption layer 401 be made from a-Si andlet the refraction index n1 of the light absorption layer 401 have avalue in a range of 1.4 to 3.5 (that is, 1.4≦n1≦3.5). In other words,let the refraction index n1 of the light absorption layer 401 have atypical value of 3.0. In addition, let the gate insulation film 110 bemade from a silicon oxide and the refraction index n2 of the gateinsulation film 110 have a value of 1.4 (that is, n2=1.4). In this case,the light-reflection preventing film 301 is created from a material withits refraction index n0 of about 2.0.

In addition, if the light-reflection preventing film 301 is made from amaterial having a refraction index n0, it is preferable to create thelight-reflection preventing film 301 with its thickness d satisfying thefollowing equation:

d=λ/(2×n0)×(m+1/2) where m=0,1,2 and so on.

That is to say, let the refraction index n1 of a material used formaking the light absorption layer 401 have a value of 2.0. In this case,in order to prevent light having a typical wavelength λ of 800 nm frombeing reflected for example, from the above equation, thelight-reflection preventing film 301 is so created that the thickness dof the light-reflection preventing film 301 is equal to 100 nm (form=0). As an alternative, the light-reflection preventing film 301 is socreated that the thickness d of the light-reflection preventing film 301is equal to 300 nm (for m=1) or 500 nm (for m=2).

Then, after a film creation process for the light absorption layer 401is carried out under typical conditions described below, a patterncreation work is performed in order to create the light absorption layer401.

Conditions

Material: Amorphous silicon

Thickness: 50 to 1,000 nm

Film creation method: CVD method

(d) Creation of the Wiring Layer 111 (FIG. 9D)

Then, the wiring layer 111 is created as shown in FIG. 9D.

In this process, the wiring layer 111 is created so as to cover an uppersurface shown in FIG. 9D as the front surface of the semiconductorsubstrate 101 in which members including the light absorption layer 401have been provided.

(e) Creation of Other Members

Then, on a lower surface shown in FIG. 3 but on an upper surface shownin FIG. 9D as the front surface of the wiring layer 111, the supportsubstrate SS is properly pasted as shown in FIG. 3. Subsequently, a filmthinning process is carried out on the semiconductor substrate 101. Forexample, a CMP (Chemical Mechanical Polishing) process is carried out onthe rear surface of the semiconductor substrate 101 in order to decreasethe thickness of the semiconductor substrate 101, for example, from 2.0μm to 1.0 μm. In FIG. 3, the rear surface of the semiconductor substrate101 is an upper surface.

It is to be noted that members such as the photodiode 21 and the pixeltransistor circuit Tr are created also on a semiconductor layer of anSOI (Silicon on Insulator) substrate shown in none of the figures and,in the same say as what has been described above, a film thinningprocess may be carried out after the wiring layer 111 and the supportsubstrate SS have been provided.

Then, as shown in FIG. 3, the light shielding layer 122, the colorfilter CF and the microlens ML are created.

By sequentially carrying out the above processes as explained so far, aCMOS image sensor of the rear-surface radiation type is completed. It isto be noted that the execution order of the process of creating thepixel transistor circuit Tr including the transfer transistor 22 and theprocesses of creating the light-reflection preventing film 301 as wellas the light absorption layer 401 can be reversed.

(C) Conclusions

As described above, in this embodiment, the photodiode 21 for receivingincident light H and generating signal electric charge from the incidentlight H is provided in the semiconductor substrate 101. The incidentlight H includes light in the visible-light domain. The photodiode 21 isconfigured to receive the light in the visible-light domain and generatethe signal electric charge from the light. In addition, the pixeltransistor circuit Tr is provided on a side close to a lower surfaceshown in FIG. 3 as the front surface of the semiconductor substrate 101.The lower surface shown in FIG. 3 as the front surface of thesemiconductor substrate 101 is a surface on a side opposite to an uppersurface shown in FIG. 3 as the rear surface of the semiconductorsubstrate 101. The upper surface shown in FIG. 3 as the rear surface ofthe semiconductor substrate 101 is a surface at which the incident lightH arrives. The pixel transistor circuit Tr is configured to output thesignal electric charge, which has been generated by the photodiode 21,as an electrical signal. Then, as shown in FIG. 3, the wiring layer 111including the wires 111 h connected to the pixel transistor circuit Tris provided so as to cover the pixel transistor circuit Tr on the sideclose to the front surface of the semiconductor substrate 101.

In the solid-state imaging device 1 having the rear-surface radiationtype, the incident light H coming from a light source provided above thesolid-state imaging device 1 may pass through the semiconductorsubstrate 101 in some cases. Then, the incident light H passing throughthe semiconductor substrate 101 may be reflected by the wires 111 h ofthe wiring layer 111 and may arrive again at the photodiode 21 of thesemiconductor substrate 101 in some cases. Since the light arrivingagain at the photodiode 21 may cause the photodiode 21 to generatesignal electric charge in some cases, a signal representing the signalelectric charge may contain noises so that the quality of the takenimage may deteriorate in some cases.

In the case of this embodiment, however, as shown in FIG. 3, on a sideclose to a lower surface shown in the figure as the front surface of thesemiconductor substrate 101, the light absorption layer 401 is provided.In this configuration shown in FIG. 3, the light absorption layer 401 isprovided between the wiring layer 111 and a portion included in a lowersurface shown in the figure as the front surface of the semiconductorsubstrate 101 as a portion at which the photodiode 21 is created so thatlight passing through the semiconductor substrate 101 as part of theincident light H is absorbed by the light absorption layer 401.

In the case of this embodiment, the light absorption layer 401 is madefrom the amorphous silicon. The amorphous silicon has a light absorptioncoefficient greater than that of the single-crystal silicon, from whichthe semiconductor substrate 101 is made, by at least one digit.

To put it concretely, the following description compares the lightabsorption coefficient of the amorphous silicon with that of thesingle-crystal silicon. In addition, the following description alsocompares the fundamental absorption edge of the amorphous silicon withthat of the single-crystal silicon.

The light absorption coefficients of the amorphous silicon are given asfollows:

200 cm⁻¹ for an incident-light wavelength λ of 800 nm

8,000 cm⁻¹ for an incident-light wavelength λ of 650 nm

60,000 cm⁻¹ for an incident-light wavelength λ of 540 nm

The light absorption coefficients of the single-crystal silicon aregiven as follows:

1,300 cm⁻¹ for an incident-light wavelength λ of 800 nm

4,500 cm⁻¹ for an incident-light wavelength λ of 650 nm

10,000 cm⁻¹ for an incident-light wavelength λ of 540 nm

The fundamental absorption edge of the amorphous silicon:

690 nm (a band gap of 1.8 eV)

The fundamental absorption edge of the single-crystal silicon:

1,100 nm (a band gap of 1.12 eV)

Thus, for a film thickness of 600 nm for example, the quantity of thereflection of red-color light from the wiring layer 111 to the lightreceiving section is reduced to a fraction equal to or smaller than 1/10in comparison with a configuration having no light absorption layer 401.

The light absorption layer 401 is created so as to absorb more light inthe visible-light domain than the semiconductor substrate 101. Thus, thelight absorption layer 401 absorbs light propagating to the wiring layer111 through the semiconductor substrate 101 before the light arrives atthe wiring layer 111.

Accordingly, since the embodiment is capable of preventing light frombeing reflected by the wires 111 h included in the wiring layer 111, thequality of the taken image can be improved.

In addition, the embodiment also includes the light-reflectionpreventing film 301 provided between a lower surface shown in FIG. 3 asthe front surface of the semiconductor substrate 101 and the lightabsorption layer 401. Thus, the embodiment is capable of preventing theincident light H passing through the semiconductor substrate 101 frombeing reflected by the boundary face between a lower surface shown inFIG. 3 as the front surface of the semiconductor substrate 101 and thelight absorption layer 401. As a result, since the light absorptionlayer 401 is capable of absorbing light with a high degree ofefficiency, the quality of the taken image can be further improved.

(D) Modified Versions (D-1) First Modified Version

In the case of the first embodiment described above, the lightabsorption layer 401 is made from undoped amorphous silicon. However,materials used for making the light absorption layer 401 are by no meanslimited to the undoped amorphous silicon. That is to say, the lightabsorption layer 401 can also be made from a material other than theundoped amorphous silicon.

In the case of pure amorphous silicon, the band gap has a valueapproximately in a range of 1.4 eV to 1.8 eV. For a band gap of 1.8 eV,the fundamental absorption edge has a value of about 690 nm. Thus, ifthe light absorption layer 401 is made from amorphous silicon, lighthaving a large wavelength is not absorbed that much. A typical exampleof the light having a large wavelength is light in the infrared domain.In addition, the single-crystal silicon absorbs incident light with asmall wavelength to a small depth from a surface hit by the light.However, the single-crystal silicon absorbs incident light with a largewavelength deeply from a surface hit by the light. Thus, light having alarge wavelength propagates to the wiring layer 111 after passingthrough the semiconductor substrate 101 made from the single-crystalsilicon. The light is then reflected by the wires 111 h of the wiringlayer 111 so that the quality of the taken image deteriorates.

However, the light absorption layer 401 is made from the amorphoussilicon doped with typically Ge or the like in order to make it possibleto set the band gap at a value close to a value of 1.1 eV which is theband gap of the single-crystal silicon and to set the fundamentalabsorption edge at a value corresponding to a larger wavelength.

In this case, it is preferable to create the light absorption layer 401under typical conditions described below.

Ge Used as a Dopant

The concentration of Ge in the amorphous silicon is in a typical rangeof 10 to 30 at. %

In this case, the value of the fundamental absorption edge is 890 nm fora band gap of 1.4 eV whereas the value of the light absorptioncoefficient is 10,000 cm⁻¹ for a wave length of 800 nm or 60,000 cm⁻¹for a wave length of 650 nm.

In addition, even if the light absorption layer 401 is made frommicrocrystal silicon, like what is described above, it is possible toset the fundamental absorption edge at a value corresponding to a largerwavelength.

In this case, it is preferable to create the light absorption layer 401under typical conditions described below.

Light Absorption Layer 401 Made from Small-Crystal Silicon

The crystal particle radius: 4 to 100 nm

Thickness: 50 to 1,000 nm

Light absorption coefficient of microcrystal silicon:

1,300 cm⁻¹ for an incident-light wavelength λ of 800 nm

6,000 cm⁻¹ for an incident-light wavelength λ of 650 nm

20,000 cm⁻¹ for an incident-light wavelength λ of 540 nm

Fundamental absorption edge of microcrystal silicon: 1,100 nm (a bandgap of 1.12 eV)

Thus, this modified version is capable of further improving the qualityof the taken image. It is to be noted that, since materials cited in thedescription of this modified version can be manufactured with ease bymaking use of semiconductor-device manufacturing facilities, effects ofthe use of the materials on the capital investment and the manufacturingcost are small.

(D-2) Second Modified Version

FIG. 10 is a diagram showing principal elements of a solid-state imagingdevice according to a second modified version of the first embodiment.

In the same way as FIG. 5, FIG. 10 is a diagram showing principalelements included in the solid-state imaging device as elements on aside close to a lower surface shown in FIG. 3 as the front surface ofthe semiconductor substrate 101.

As shown in FIG. 10, a contact 401C can be provided on the lightabsorption layer 401. That is to say, it is possible to provide aconfiguration in which the light absorption layer 401 and the wires 111h included in the wiring layer 111 as shown in FIG. 3 are electricallyconnected to each other through the contact 401C.

In the case of such a configuration, it is preferable to create thelight absorption layer 401 from typically microcrystal silicon dopedwith B so as to make the light absorption layer 401 conductive.

Then, a voltage is applied to the light absorption layer 401 through thewires 111 h connected to the light absorption layer 401 so as to preventa dark current from being generated on a lower surface shown in FIG. 3as the front surface of the semiconductor substrate 101.

In the case of this modified version, a typical negative voltage in arange of −2.7 to −1.0 V is applied to the light absorption layer 401. Toput it in detail, the negative voltage is applied to the lightabsorption layer 401 throughout an entire time period shown in FIGS. 8Ato 8C as a time period between the times t1 and t5. During this timeperiod, all operations are carried out sequentially. The operationsinclude a signal electric-charge accumulation period, a signalelectric-charge read period and a reset period. Thus, holes areaccumulated in the vicinity of a lower surface shown in FIG. 3 as thefront surface of the semiconductor substrate 101. As a result, it ispossible to prevent a dark current from being generated due to adangling bond effect on a boundary face on a lower surface shown in FIG.3 as the front surface of the semiconductor substrate 101.

It is to be noted that, during the signal electric-charge read operationcarried out in a time period between the times t4 and t5 shown in FIGS.8A to 8C, a positive voltage may be applied to the light absorptionlayer 401 temporarily. In this case, the positive voltage applied to thelight absorption layer 401 is lower than a positive voltage applied tothe gate electrode 22G of the transfer transistor 22 during theelectric-charge read period. For example, the positive voltage appliedto the gate electrode 22G of the transfer transistor 22 is +3.3 V or+2.7 V whereas the positive voltage applied to the light absorptionlayer 401 is 1.0 V. Thus, the light absorption layer 401 causes theaccumulated signal electric charge to make a movement of approaching alower surface shown in FIG. 3 as the front surface of the semiconductorsubstrate 101. As a result, the signal electric-charge read operation toread out signal electric charge can be carried out effectively (forexample, refer to Japanese Patent Laid-Open No. 2007-258684).

Accordingly, this modified version is capable of further improving thequality of the taken image.

2: Second Embodiment (A) Device Configuration

FIGS. 11 and 12 are each a diagram showing principal elements of asolid-state imaging device according to a second embodiment.

Much like FIG. 3, FIG. 11 is a diagram showing a cross section. Muchlike FIG. 5, on the other hand, FIG. 12 shows a side close to a lowersurface shown in FIG. 11 as the front surface of the semiconductorsubstrate 101 of the solid-state imaging device.

To put it in detail, FIG. 11 shows a model representing a cross sectionof a portion X1-X2 shown in FIG. 12. FIG. 12 shows a lower surface shownin FIG. 11 as the front surface of the semiconductor substrate 101except a support substrate SS and a wiring layer 111. It is to be notedthat, for the sake of drawing convenience, in the figures, the shape(including the width) of each portion may be properly changed fromdiagram to diagram.

As shown in FIGS. 11 and 12, a light-reflection preventing film 301 band a light absorption layer 401 b which are employed in the secondembodiment are different from their respective counterparts employed inthe first embodiment. Except for these differences and maters related tothese differences, the second embodiment is identical with the firstone. Thus, explanation of portions common to the first and secondembodiments is properly omitted from the following description.

As shown in FIG. 11, the light-reflection preventing film 301 b isprovided on a lower surface, which is shown in the figure as the frontsurface of the semiconductor substrate 101, so as to sandwich a gateinsulation film 110 between the light-reflection preventing film 301 band the semiconductor substrate 101.

However, the second embodiment is different from the first embodimentshown in some diagrams including FIG. 3 in that, in the case of thesecond embodiment, the light-reflection preventing film 301 b isprovided so as to cover the gate electrode 22G of the transfertransistor 22 as shown in FIG. 11. In addition, the light-reflectionpreventing film 301 b is provided so as to flatten a lower surface shownin FIG. 11 as the front surface of the semiconductor substrate 101 inwhich the gate electrode 22G of the transfer transistor 22 has beencreated.

That is to say, as shown in FIG. 11, the light-reflection preventingfilm 301 b is created to have a thickness greater than the gateelectrode 22G of the transfer transistor 22. For example, on the basisof an equation described earlier, the light-reflection preventing film301 b is created so that the thickness d of the light absorption layer401 b is 300 nm (for m=1) or 500 nm (for m=2).

As shown in FIG. 11, the light absorption layer 401 b is created on alower surface shown in the figure as the front surface of thesemiconductor substrate 101 so as to sandwich the gate insulation film110 and the light-reflection preventing film 301 b between the lightabsorption layer 401 b and the semiconductor substrate 101.

In the case of this embodiment, however, as shown in FIG. 11, the lightabsorption layer 401 b is created on a lower surface shown in the figureas the front surface of the light-reflection preventing film 301 b so asto cover also the gate electrode 22G of the transfer transistor 22. Thatis to say, the light absorption layer 401 b is provided not on the samelayer as the gate electrode 22G.

As shown in FIG. 12, the light absorption layer 401 b is created on anentire lower surface shown in FIG. 11 as the front surface of thesemiconductor substrate 101 so as to cover a specific portion other thana portion in which the contact CON has been provided. That is to say,the light absorption layer 401 b is created so as to cover a lowersurface shown in FIG. 11 as the front surface of the semiconductorsubstrate 101 but provide an aperture on the portion in which thecontact CON has been provided. The light absorption layer 401 b isprovided as a single junction between pixels P lined up in thehorizontal direction x and the vertical direction y.

(B) Conclusions

As described above, in the same way as the first embodiment, in the caseof the second embodiment, the light absorption layer 401 b is providedon a lower surface shown in FIG. 11 as the front surface of thesemiconductor substrate 101. Thus, light propagating to the wiring layer111 through the semiconductor substrate 101 is absorbed by the lightabsorption layer 401 b before the light arrives at the wiring layer 111.

Thus, the second embodiment is capable of preventing the lightpropagating to the wiring layer 111 through the semiconductor substrate101 from being reflected by the wires 111 h of the wiring layer 111. Asa result, the quality of the taken image can be improved.

It is to be noted that the concepts adopted in the first and secondmodified versions of the first embodiment can be applied to the secondembodiment.

3: Others

Implementations of the present disclosure are by no means limited to thefirst and second embodiments described so far and the first and secondmodified versions of the first embodiment. That is to say, the first andsecond embodiments as well as the first and second modified versions ofthe first embodiment can be further modified in a variety of ways inorder to implement the present disclosure.

On top of that, in the embodiments described above, the presentdisclosure is applied to cameras. However, applications of the presentdisclosure are by no means limited to cameras. That is to say, thepresent disclosure can also be applied to other electronic apparatuseach employing the solid-state imaging device according to any of theembodiments. Typical examples of the other electronic apparatus includea scanner and a copier.

In addition, in the embodiments described above, the pixel transistorcircuit Tr includes four different transistors, i.e., the transfertransistor 22, the amplification transistor 23, the select transistor 24and the reset transistor 25. However, the pixel transistor circuit Tr isby no means limited to such a configuration. For example, it is alsopossible to provide a configuration in which the pixel transistorcircuit Tr includes three different transistors, i.e., the transfertransistor 22, the amplification transistor 23 and the reset transistor25.

On top of that, in the embodiments described above, one transfertransistor 22, one amplification transistor 23, one select transistor 24and one reset transistor 25 are provided for each photodiode 21.However, implementations of the present disclosure are by no meanslimited to such a configuration. For example, it is also possible toadopt a configuration in which one amplification transistor 23, oneselect transistor 24 and one reset transistor 25 are provided for eachplurality of photo diodes 21.

In addition, the present disclosure can be applied to not only a CMOSimage sensor, but also a CCD (Charge-Coupled Device) image sensor.

On top of that, in the embodiments described above, a light-reflectionpreventing film 301 or 301 b is provided. However, implementations ofthe present disclosure are by no means limited to such a configuration.For example, it is also possible to adopt a configuration in which thelight-reflection preventing film 301 or 301 b is eliminated.

In addition, the embodiments described above can be combined properly.

That is to say, the present disclosure can be realized into thefollowing implementations.

1: A solid-state imaging device including:

an opto-electrical conversion section provided inside a semiconductorsubstrate to receive incident light coming from one surface of thesemiconductor substrate;

a wiring layer provided on the other surface of the semiconductorsubstrate; and

a light absorption layer provided between the other surface of thesemiconductor substrate and the wiring layer to absorb transmitted lightpassing through the opto-electrical conversion section as part of theincident light.

2: The solid-state imaging device according to implementation 1 whereina light-reflection preventing layer is provided between the othersurface of the semiconductor substrate and the wiring layer.

3: The solid-state imaging device according to implementation 1 or 2wherein the light absorption layer is created from a material having alight absorption coefficient greater than that of the semiconductorsubstrate.

4: The solid-state imaging device according to implementation 1 or 2wherein:

the semiconductor substrate is created from single-crystal silicon; and

the light absorption layer is created from amorphous silicon.

5: The solid-state imaging device according to implementation 1 or 2wherein:

the semiconductor substrate is created from single-crystal silicon; and

the light absorption layer is created from amorphous silicon doped withgermanium.

6: The solid-state imaging device according to implementation 1 or 2wherein:

the semiconductor substrate is created from single-crystal silicon; and

the light absorption layer is created from microcrystal silicon.

7: The solid-state imaging device according to implementation 1 or 2wherein a voltage is applied to the light absorption layer in order toprevent a dark current from being generated on the other surface of thesemiconductor substrate.

8: The solid-state imaging device according to any one ofimplementations 1 to 6, the solid-state imaging device further having apixel transistor circuit provided on the other surface of thesemiconductor substrate to output signal electric charge, which isgenerated by the opto-electrical conversion section, as an electricalsignal, wherein the wiring layer is provided on the other surface of thesemiconductor substrate to sandwich the pixel transistor circuit betweenthe wiring layer and the other surface of the semiconductor substrate.

9: A method for manufacturing a solid-state imaging device including:

providing an opto-electrical conversion section inside a semiconductorsubstrate to serve as a section for receiving incident light coming fromone surface of the semiconductor substrate;

providing a light absorption layer on the other surface of thesemiconductor substrate to serve as a layer for absorbing transmittedlight passing through the opto-electrical conversion section as part ofthe incident light; and

providing a wiring layer so as to cover the other surface pertaining tothe semiconductor substrate to serve as a surface on which the lightabsorption layer has been provided.

10: An electronic apparatus including:

an opto-electrical conversion section provided inside a semiconductorsubstrate to receive incident light coming from one surface of thesemiconductor substrate;

a wiring layer provided on the other surface of the semiconductorsubstrate; and

a light absorption layer provided between the other surface of thesemiconductor substrate and the wiring layer to absorb transmitted lightpassing through the opto-electrical conversion section as part of theincident light.

It is to be noted that, in the embodiments described above, thephotodiode 21 is a typical opto-electrical conversion section whereasthe camera 40 is a typical electronic apparatus.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2011-067076 filedin the Japan Patent Office on Mar. 25, 2011, the entire content of whichis hereby incorporated by reference.

What is claimed is:
 1. An electronic apparatus comprising: anopto-electrical conversion section provided inside a semiconductorsubstrate to receive incident light coming from one surface of saidsemiconductor substrate; a wiring layer provided on another surface ofsaid semiconductor substrate; and a light absorption layer providedbetween said another surface of said semiconductor substrate and saidwiring layer to absorb transmitted light passing through saidopto-electrical conversion section as part of said incident light,wherein said semiconductor substrate is created from single crystalsilicon, and wherein said light absorption layer is created from atleast one material selected from the group consisting of amorphoussilicon, microcrystal silicon, and amorphous silicon doped withgermanium.
 2. The electronic apparatus according to claim 1 wherein alight-reflection preventing layer is provided between said anothersurface of said semiconductor substrate and said wiring layer.
 3. Theelectronic apparatus according to claim 1 wherein said light absorptionlayer is created from a material having a light absorption coefficientgreater than that of said semiconductor substrate.
 4. The electronicapparatus according to claim 1 wherein: said light absorption layer iscreated from amorphous silicon.
 5. The electronic apparatus according toclaim 1 wherein: said light absorption layer is created from amorphoussilicon doped with germanium.
 6. The electronic apparatus according toclaim 1 wherein: said light absorption layer is created frommicrocrystal silicon.
 7. The electronic apparatus according to claim 1wherein a voltage is applied to said light absorption layer in order toprevent a dark current from being generated on said another surface ofsaid semiconductor substrate.
 8. The electronic apparatus according toclaim 1, said electronic apparatus further comprising a pixel transistorcircuit provided on said another surface of said semiconductor substrateto output signal electric charge, which is generated by saidopto-electrical conversion section, as an electrical signal, whereinsaid wiring layer is provided on said another surface of saidsemiconductor substrate to sandwich said pixel transistor circuitbetween said wiring layer and said another surface of said semiconductorsubstrate.